Browsing by Author "Shah, Ali"
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- Black silicon technology and applications
School of Electrical Engineering | Doctoral dissertation (article-based)(2016) Shah, AliThis thesis focuses on fabrication, optimisation and integration of black silicon (bSi) for different applications. The research work presented in this thesis is divided into two parts. In the first part, bSi formation was studied in an inductively coupled plasma-reactive ion etcher (ICP-RIE). The design of experiments (DOE) technique was used to evaluate the influence of process parameters on bSi formation. The outcome was used to establish guidelines for fabricating different types of bSi. Applications of bSi are discussed in the second part of this thesis. Process development using standard and novel micro and nanofabrication techniques was performed to enable bSi employment for targeted applications. The developed processes were used to achieve patterned wetting of liquid droplets and a wide band optical enhancement. For patterned wetting, novel fabrication processes were developed to achieve patterns that composed extreme wetting contrast with the substrate. Hydrophobic, hydrophilic and superhydrophilic patterns were fabricated with superhydrophobic surroundings. Upon dispensing, the liquid droplets confined to more wettable patterns and mimicked their shape. Due to an extreme wetting contrast and topographical discontinuity, patterned wetting to a large number of patterns was achieved. A fabricated template containing patterns with extreme wetting contrast and topographical discontinuity with the surrounding substrate was used to demonstrate self-alignment of microchips. High accuracy, reliable and repeatable self-alignment of microchips was recorded. Several techniques were employed to improve the self-alignment of microchips on bSi based self-alignment template. Self-alignment of microchips is an increasingly popular technique for advanced packaging. Optical enhancement was achieved by optimisation of bSi surface structures. Improved anti-reflection and light trapping behaviour were demonstrated in UV-VIS spectrum. In order to extend the anti-reflection behaviour of bSi beyond UV-VIS, conformal pyrolytic carbon (PyC) coating was deposited and a substrate with exceptionally low reflectance over a wide spectrum (UV-NIR) was achieved. The surface structure optimisation was also exploited for plasmonic enhancement. Thin silver (Ag) films and different bSi surface structures were studied to achieve highly sensitive surface-enhanced Raman spectroscopy (SERS) substrate. - Direct transfer of Wafer-scale graphene films
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-09-01) Kim, Maria; Shah, Ali; Li, Changfeng; Mustonen, Petri; Susoma, Jannatul; Manoocheri, Farshid; Riikonen, Juha; Lipsanen, HarriFlexible electronics serve as the ubiquitous platform for the next-generation life science, environmental monitoring, display, and energy conversion applications. Outstanding multifunctional mechanical, thermal, electrical, and chemical properties of graphene combined with transparency and flexibility solidifies it as ideal for these applications. Although chemical vapor deposition (CVD) enables cost-effective fabrication of high-quality large-area graphene films, one critical bottleneck is an efficient and reproducible transfer of graphene to flexible substrates. We explore and describe a direct transfer method of 6-inch monolayer CVD graphene onto transparent and flexible substrate based on direct vapor phase deposition of conformal parylene on as-grown graphene/copper (Cu) film. The method is straightforward, scalable, cost-effective and reproducible. The transferred film showed high uniformity, lack of mechanical defects and sheet resistance for doped graphene as low as 18 Ω/sq and 96.5% transparency at 550 nm while withstanding high strain. To underline that the introduced technique is capable of delivering graphene films for next-generation flexible applications we demonstrate a wearable capacitive controller, a heater, and a self-powered triboelectric sensor. - Macro-, micro-and nano-roughness of carbon-based interface with the living cells
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-09-02) Golubewa, Lena; Rehman, Hamza; Kulahava, Tatsiana; Karpicz, Renata; Baah, Marian; Kaplas, Tommy; Shah, Ali; Malykhin, Sergei; Obraztsov, Alexander; Rutkauskas, Danielis; Jankunec, Marija; Matulaitienė, Ieva; Selskis, Algirdas; Denisov, Andrei; Svirko, Yuri; Kuzhir, PolinaIntegration of living cells with nonbiological surfaces (substrates) of sensors, scaffolds, and implants implies severe restrictions on the interface quality and properties, which broadly cover all elements of the interaction between the living and artificial systems (materials, surface modifications, drug-eluting coatings, etc.). Substrate materials must support cellular viability, preserve sterility, and at the same time allow real-time analysis and control of cellular activity. We have compared new substrates based on graphene and pyrolytic carbon (PyC) for the cultivation of living cells. These are PyC films of nanometer thickness deposited on SiO2 and black silicon and graphene nanowall films composed of graphene flakes oriented perpendicular to the Si substrate. The structure, morphology, and interface properties of these substrates are analyzed in terms of their biocompatibility. The PyC demonstrates interface biocompatibility, promising for controlling cell proliferation and directional intercellular contact formation while as-grown graphene walls possess high hydrophobicity and poor biocompatibility. By performing experiments with C6 glioma cells we discovered that PyC is a cell-friendly coating that can be used without poly-l-lysine or other biopolymers for controlling cell adhesion. Thus, the opportunity to easily control the physical/chemical properties and nanotopography makes the PyC films a perfect candidate for the development of biosensors and 3D bioscaffolds. - Maskless, High-Precision, Persistent, and Extreme Wetting-Contrast Patterning in an Environmental Scanning Electron Microscope
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016) Liimatainen, Ville; Shah, Ali; Johansson, Leena-Sisko; Houbenov, Nikolay; Zhou, Quan - Nonlinear plasmonic behavior of nanohole arrays in thin gold films for imaging lipids
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-06-04) Subramaniyam, Nagarajan; Shah, Ali; Dreser, Christoph; Isomäki, Antti; Fleischer, Monika; Sopanen, MarkkuWe demonstrate linear and nonlinear plasmonic behaviors of periodic nanohole arrays in thin gold (Au) films with varying periodicities. As expected, the linear optical transmission spectra of the nanohole arrays show a red-shift of the resonance wavelength and Wood's anomaly with increasing hole spacing. The optical transmission and electric near-field intensity distribution of the nanohole arrays are simulated using the finite element method. The nonlinear plasmonic behavior of the nanohole arrays is studied by using picosecond pulsed excitation at near-infrared wavelengths. The characteristic nonlinear signals indicating two-photon excited luminescence (TPEL), sum frequency generation, second harmonic generation, and four-wave mixing (FWM) are observed. A maximum FWM/TPEL signal intensity ratio is achieved for nanohole arrays with a periodicity of 500 nm. Furthermore, the significant FWM signal intensity and contrast compared to the background were harnessed to demonstrate the ability of surface-enhanced coherent anti-Stokes Raman scattering to visualize low concentrations of lipids deposited on the nanohole array with a periodicity of 500 nm. - Oil droplet self-transportation on oleophobic surfaces
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-06-17) Li, Juan; Qin, QiHang; Shah, Ali; Ras, Robin; Tian, Xuelin; Jokinen, VilleDirectional liquid transportation is important for a variety of biological processes and technical applications. Although surface engineering through asymmetric chemical modification or geometrical patterning facilitates effective liquid manipulation and enables water droplet self-transportation on synthetic surfaces, self-transportation of oil droplets poses a major challenge because of their low surface tension. We report oil droplet self-transportation on oleophobic surfaces that are microtextured with radial arrays of undercut stripes. More significantly, we observe three modes of oil motion on various sample surfaces, namely, inward transportation, pinned, and outward spreading, which can be switched by the structure parameters, including stripe intersection angle and width. Accompanying theoretical modeling provides an in-depth mechanistic understanding of the structure–droplet motion relationship. Finally, we reveal how to optimize the texture parameters to maximize oil droplet self-transportation capability and demonstrate spontaneous droplet movement for liquids down to a surface tension of 22.4 mN/m. The surfaces presented here open up new avenues for power-free liquid transportation and oil contamination self-removal applications in various analytical and fluidic devices. - Pyrolytic carbon coated black silicon
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-05-13) Shah, Ali; Stenberg, Petri; Karvonen, Lasse; Ali, Rizwan; Honkanen, Seppo; Lipsanen, Harri; Peyghambarian, N.; Kuittinen, Markku; Svirko, Yuri; Kaplas, TommiCarbon is the most well-known black material in the history of man. Throughout the centuries, carbon has been used as a black material for paintings, camouflage, and optics. Although, the techniques to make other black surfaces have evolved and become more sophisticated with time, carbon still remains one of the best black materials. Another well-known black surface is black silicon, reflecting less than 0.5% of incident light in visible spectral range but becomes a highly reflecting surface in wavelengths above 1000 nm. On the other hand, carbon absorbs at those and longer wavelengths. Thus, it is possible to combine black silicon with carbon to create an artificial material with very low reflectivity over a wide spectral range. Here we report our results on coating conformally black silicon substrate with amorphous pyrolytic carbon. We present a superior black surface with reflectance of light less than 0.5% in the spectral range of 350 nm to 2000 nm. - Self-transport and self-alignment of microchips using microscopic rain
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015) Chang, Bo; Shah, Ali; Zhou, Quan; Ras, Robin; Hjort, KlasAlignment of microchips with receptors is an important process step in the construction of integrated micro- and nanosystems for emerging technologies, and facilitating alignment by spontaneous self-assembly processes is highly desired. Previously, capillary self-alignment of microchips driven by surface tension effects on patterned surfaces has been reported, where it was essential for microchips to have sufficient overlap with receptor sites. Here we demonstrate for the first time capillary self-transport and self-alignment of microchips, where microchips are initially placed outside the corresponding receptor sites and can be self-transported by capillary force to the receptor sites followed by self-alignment. The surface consists of hydrophilic silicon receptor sites surrounded by superhydrophobic black silicon. Rain-induced microscopic droplets are used to form the meniscus for the self-transport and self-alignment. The boundary conditions for the self-transport have been explored by modeling and confirmed experimentally. The maximum permitted gap between a microchip and a receptor site is determined by the volume of the liquid and by the wetting contrast between receptor site and substrate. Microscopic rain applied on hydrophilic-superhydrophobic patterned surfaces greatly improves the capability, reliability and error-tolerance of the process, avoiding the need for accurate initial placement of microchips, and thereby greatly simplifying the alignment process. - Self-transport and self-alignment of microchips using microscopic rain
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015-10-09) Chang, Bo; Shah, Ali; Zhou, Quan; Ras, Robin H. A.; Hjort, KlasAlignment of microchips with receptors is an important process step in the construction of integrated micro- and nanosystems for emerging technologies, and facilitating alignment by spontaneous self-assembly processes is highly desired. Previously, capillary self-alignment of microchips driven by surface tension effects on patterned surfaces has been reported, where it was essential for microchips to have sufficient overlap with receptor sites. Here we demonstrate for the first time capillary self-transport and self-alignment of microchips, where microchips are initially placed outside the corresponding receptor sites and can be self-transported by capillary force to the receptor sites followed by self-alignment. The surface consists of hydrophilic silicon receptor sites surrounded by superhydrophobic black silicon. Rain-induced microscopic droplets are used to form the meniscus for the self-transport and self-alignment. The boundary conditions for the self-transport have been explored by modeling and confirmed experimentally. The maximum permitted gap between a microchip and a receptor site is determined by the volume of the liquid and by the wetting contrast between receptor site and substrate. Microscopic rain applied on hydrophilic-superhydrophobic patterned surfaces greatly improves the capability, reliability and error-tolerance of the process, avoiding the need for accurate initial placement of microchips, and thereby greatly simplifying the alignment process. - Sliding droplets on hydrophilic/superhydrophobic patterned surfaces for liquid deposition
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-04-11) Chang, Bo; Zhou, Quan; Ras, Robin H A; Shah, Ali; Wu, Zhigang; Hjort, KlasA facile gravity-induced sliding droplets method is reported for deposition of nanoliter sized droplets on hydrophilic/superhydrophobic patterned surface. The deposition process is parallel where multiple different liquids can be deposited simultaneously. The process is also high-throughput, having a great potential to be scaled up by increasing the size of the substrate. - Surface-enhanced Raman spectroscopy of organic molecules and living cells with gold-plated black silicon
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-11-11) Golubewa, Lena; Karpicz, Renata; Matulaitiene, Ieva; Selskis, Algirdas; Rutkauskas, Danielis; Pushkarchuk, Aliaksandr; Khlopina, Tatsiana; Michels, Dominik; Lyakhov, Dmitry; Kulahava, Tatsiana; Shah, Ali; Svirko, Yuri; Kuzhir, PolinaBlack silicon (bSi) refers to an etched silicon surface comprising arrays of microcones that effectively suppress reflection from UV to near-infrared (NIR) while simultaneously enhancing the scattering and absorption of light. This makes bSi covered with a nmthin layer of plasmonic metal, i.e., gold, an attractive substrate material for sensing of bio-macromolecules and living cells using surface-enhanced Raman spectroscopy (SERS). The performed Raman measurements accompanied with finite element numerical simulation and density functional theory analysis revealed that at the 785 nm excitation wavelength, the SERS enhancement factor of the bSi/Au substrate is as high as 108 due to a combination of electromagnetic and chemical mechanisms. This finding makes the SERS-active bSi/Au substrate suitable for detecting trace amounts of organic molecules. We demonstrate the outstanding performance of this substrate by highly sensitive and specific detection of a small organic molecule of 4-mercaptobenzoic acid and living C6 rat glioma cell nucleic acids/proteins/lipids. Specifically, the bSi/Au SERS-active substrate offers a unique opportunity to investigate the living cells' malignant transformation using characteristic protein disulfide Raman bands as a marker. Our findings evidence that bSi/Au provides a pathway to the highly sensitive and selective, scalable, and low-cost substrate for lab-on-a-chip SERS biosensors that can be integrated into silicon-based photonics devices. - Surface-tension driven self-assembly of microchips on hydrophobic receptor sites with water using forced wetting
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Chang, Bo; Shah, Ali; Routa, Iiris; Lipsanen, Harri; Zhou, QuanThis letter reports waterdroplet self-alignment methods for self-assembly of microchips on hydrophobic receptor sites in ambient air environment. It is an open question if lyophobic receptor site of the self-alignment medium can be used for self-assembly. We investigate this question using both numerical simulation and experimental studies on hydrophobic receptor sites (advancing contact angle of 118°) with superhydrophobic substrate (contact angle of 180°). We demonstrate that self-alignment is possible using two forced wetting methods: (a) introducing an excessive amount of water and (b) applying external pressure. The results suggest that surface-tension driven self-alignment can be applied in a wider combination of materials and mediums. - Wide-Band Black Silicon with Atomic Layer Deposited NbN
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-06-11) Isakov, Kirill; Pyymaki Perros, Alexander; Shah, Ali; Lipsanen, HarriAntireflection surfaces are often utilized in optical components to reduce undesired reflection and increase absorption. We report on black silicon (b-Si) with dramatically enhanced absorption over a broad wavelength range (250– 2500 nm) achieved by applying a 10–15 nm conformal coating of NbN with atomic layer deposition (ALD). The improvement is especially pronounced in the near infrared (NIR) range of 1100– 2500 nm where absorption is increased by >90%. A significant increase of absorption is also observed over the ultraviolet (UV) range of 200–400 nm. Preceding NbN deposition with a nanostructured ALD Al2O3 (n-Al2O3) coating to enhance the NbN texture was also examined. Such texturing further improves absorption in the NIR, especially at longer wavelengths, strong absorption up to 4–5 μm wavelengths has been attested. For comparison, double side polished silicon and sapphire coated with 10 nm-thick NbN exhibited absorption of only ~55% in the NIR range of 1100–2500 nm. The results suggest a positive correlation between the surface area of NbN coating and optical absorption. Based on the wide-band absorption, the presented NbN-coated b-Si may be an attractive candidate for use in e.g. spectroscopic systems, infrared microbolometers.